Abstract: A seeker optical system of nonspherical dome with large fineness ratio is purposed in this paper. Properties of the aberration and its correction methods are analyzed and discussed. The designed dome optical system has the following features: fineness ratio is 2, operating waveband is the middle infrared spectrum, half of the field is 30 degrees, and its <em<F</em< number is 2. The results show that the image quality is approaching the diffraction limit and satisfies the requirements of imaging optical system.